Mice infected with LP-BM5 murine leukemia viruses (MuLV) develop a syndrome of lymphadenopathy and profound immunosuppression with many immunologic features in common with AIDS. Development of this disease is completely dependent on the activity of T cells since infected nude mice do not develop B cell abnormalities. In addition, it has been found that development of disease is inhibited in mice depleted of Ly-4+ T cells before infection and that many aspects of disease are reversed in animals depleted of Ly-4+ cells after infection. The late stages of LP-BM-5 induced disease are associated with enhanced susceptability to infection and the development of B cell lymphomas; C57BL/6 mice infected with LP-BM5 MuLV become susceptible to lethal infection with ectromelia virus and oligoclonal or monoclonal proliferations of B cells are detected in lymphoid tissues of almost all moribund mice.